1. Field of the Invention
The present invention generally relates to a Multi Input Multi Output (MIMO) system. More particularly, the present invention relates to an apparatus and method for Automatic Repeat reQuest (ARQ) in a MIMO system.
2. Description of the Related Art
In MIMO systems, data is transmitted by using either spatial multiplexing or space-time coding. When using the spatial multiplexing, two or more data streams are simultaneously transmitted through a plurality of antennas included in a transmitting end and a receiving end. When using the space-time coding, a suitable code is combined with a data stream, and the resultant data stream is transmitted through the plurality of antennas over certain time periods. For example, in a 2×2 MIMO system, a transmitting end has two transmitting antennas and signals are transmitted through the two transmitting antennas. Then, a receiving end receives the signals so two data streams can be transmitted and received within one time period. When using an Alamouti code, the transmitting end transmits two data streams within two time periods through the two transmitting antennas, and thus a diversity gain can be obtained, thereby reducing an error rate.
According to an ARQ scheme, a transmitting end retransmits data when an error is detected from the data while the data is received by a receiving end. In particular, a Hybrid ARQ (HARQ) scheme combines an ARQ scheme and a coding scheme. With a HARQ scheme, when an error is detected from data transmitted in a previous time period, the erroneous data is stored rather than discarded, and when retransmitted, the data is combined with the previously transmitted data. This can increase probability of successful data reception. To detect errors, data packets are transmitted using a Cyclic Redundancy Check (CRC) code.
The receiving end detects errors by using a CRC code from a received packet and feeds back an acknowledgement/non-acknowledgement (ACK/NACK) signal to the transmitting end. Specifically, if no error is detected from the received packet, the ACK signal is fed back. Otherwise, the NACK signal is fed back. The transmitting end transmits a new packet to the receiving end in response to the ACK signal, and transmits the error detected signal to the receiving end in response to the NACK signal.
Unlike a Single Input Single Output (SISO) system, a MIMO system transmits two or more data streams at the same time. Therefore, in addition to an ACK/NACK signal, a stream identifier bit is also fed back to distinguish an error detected data stream. In the receiving end, a receiving filter is configured by using a pilot signal according to an antenna. When a signal is transmitted from an antenna, an output unit of the receiving filter can recognize the antenna from which the signal is transmitted. Therefore, the receiving end can recognize an error status of data streams.
In a conventional MIMO system employing an ARQ scheme, when an error is detected from a packet, the packet is retransmitted through the previously used antenna without having to select another antenna. Alternatively, all packets are retransmitted including the error detected packet by using another antenna. Since there is no method for selecting a specific antenna assigned to an error detected packet, a need exists for a method in which the error detected packet is retransmitted by selecting an antenna assigned for that packet.